You really need 5V at low current at the USB interface connector. It's a lot cheaper and easier to buy the solution. I've designed them, but not as small and as inexpensive as an eBay/Amazon/Radio Shack solution. Ours were for ruggedized applications - so couldn't use aftermarket.
There are quite a few USB "power packs" that are Li-Ion based and have charging circuitry and output boost circuitry. (Li-Ion is nominally 4.2V for a single cell, needs a boost for 5V output).
Hope that's helpful. Check out Texas Instruments / Monolithic Power Systems for switchers/Battery management IC's to roll your own.
Or here's one for 1 USD on Amazon. Could be garbage. :smileyhappy: Most decent ones are like $15. Good luck.
http://www.amazon.com/iMeshbean%C2%AE-Lipstick-Portable-Battery-Charger/dp/B00IMQYEN8/ref=sr_1_124?s...
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